1. Field of the Invention
The present invention relates to a liquid crystal display device having an orientation controller which divides a direction of alignment of liquid crystal within a pixel region.
2. Description of the Related Art
Because liquid crystal display devices (hereinafter simply referred to as “LCD”) have advantages such as a thin thickness and low power consumption, the LCDs are widely in use as a computer monitor and a monitor for a portable information device or the like. In the LCD, liquid crystal is sealed between a pair of substrates and display is realized by controlling, using electrodes formed on the substrates, alignment of the liquid crystal positioned between the electrodes.
TN (Twisted Nematic) liquid crystal is known as the liquid crystal in such an LCD. In the LCD which uses the TN liquid crystal, an alignment film to which a rubbing process is applied is formed on a contact surface, which faces the liquid crystal, of each of the pair of substrates. When no voltage is applied, the TN liquid crystal which has a positive dielectric constant anisotropy is initially aligned such that the major axis of the molecules is aligned along the direction of rubbing of the alignment film. In many cases, the initial alignment of the liquid crystal is not completely along the plane of the substrate, but a pretilt is applied in advance. That is, the major axis of the molecule is tilted by a predetermined angle from the plane of the substrate.
The rubbing direction of the alignment film on one substrate and the rubbing direction of the alignment film on the other substrate are configured so that the rubbing directions are 90° twisted from each other and the liquid crystal is aligned with a twist of 90° between the pair of substrates. When a voltage is applied to the liquid crystal between the electrodes by the electrodes formed on the opposing surfaces of the pair of substrates, the major axis direction of the liquid crystal molecule is changed toward the direction of normal of the plane of the substrate and the state of the twisted alignment is resolved.
Linear polarizer plates having polarization axes that are perpendicular to each other are provided on the pair of substrates. The rubbing direction of the alignment film is set along the direction of the polarization axis of the polarizer plate on the corresponding substrate. Because of this structure, when no voltage is applied, linearly polarized light entering the liquid crystal layer through a polarizer plate on the side of the substrate placed near a light source becomes, in the liquid crystal layer which is aligned with the twist of 90°, linearly polarized light having the polarization axis different by 90°. The converted linearly polarized light transmits through the polarizer plate which is provided on the side of the other substrate and which allows transmission of only linearly polarized light having the polarization axis at a direction 90° different from that of the polarizer plate at the side of entrance of the light. Thus, the light from the light source transmits through the LCD and “white” is displayed. When, on the other hand, a voltage is applied between the electrodes so that the twisted alignment of the liquid crystal is completely resolved and the liquid crystal molecules are aligned with the direction of normal of the plane of the substrate, the linearly polarized light entering the liquid crystal layer from the side near the light source reaches the polarizer plate provided on the other substrate without a change in the polarization in the liquid crystal layer, and thus, the polarization does not match the polarization axis of the linearly polarized light of the polarizer plate on the emission side, the light cannot transmit through the polarizer plate on the emission side, and “black” is displayed. Gray scales are expressed by adjusting the amount of light which can transmit through the polarizer plate at the emission side through application, to the liquid crystal layer, of a voltage which does not completely resolve the twisted alignment of the liquid crystal layer to convert a portion of the linearly polarized light entering the liquid crystal layer to the linearly polarized light having the polarization axis which is 90° different.
An LCD which uses a vertically aligned (VA) liquid crystal (hereinafter simply referred to as “VA liquid crystal”) is also known in addition to the TN liquid crystal. In the VA liquid crystal, the liquid crystal has, for example, a negative dielectric constant anisotropy and the major axis of the liquid crystal molecule is directed along a vertical direction (direction of normal of the plane of the substrate) when no voltage is applied because of a vertical alignment film. In an LCD which uses the VA liquid crystal, polarizer plates having polarization axes different from each other by 90° are provided on the pair of substrates. When no voltage is applied, linearly polarized light entering the liquid crystal layer through the polarizer plate on the side of the substrate placed near the light source reaches the polarizer plate on the substrate on the viewing side without a change in the polarization state because the liquid crystal is vertically aligned and birefringence does not occur in the liquid crystal layer. Thus, the light cannot transmit through the polarizer plate on the viewing side and “black” is displayed. When a voltage is applied between the electrodes, the VA liquid crystal changes so that the major axis of the molecule is tilted towards the direction of plane of the substrate. Because the VA liquid crystal has a negative optical anisotropy (index of refraction anisotropy), the minor axis of the liquid crystal molecule is tilted toward the direction of normal of the plane of the substrate and the linearly polarized light entering the liquid crystal layer from the side of the light source is changed by birefringence in the liquid crystal layer so that the linearly polarized light becomes elliptically polarized as the light transmits through the liquid crystal layer. The elliptically polarized light further becomes circularly polarized light, elliptically polarized light, or linearly polarized light (all of the polarized light has the polarization axis 90° different from the linearly polarized light which enters the liquid crystal). Because of this configuration, when all of the entering linearly polarized light becomes linearly polarized light which is different by 90° due to birefringence in the liquid crystal layer, all of the linearly polarized light transmits through the polarizer plate on the substrate on the viewing side, and the display becomes “white (maximum brightness)”. The amount of birefringence is determined by a degree of tilt of the liquid crystal molecule. Therefore, depending on the amount of birefringence, the entering linearly polarized light becomes elliptically polarized light having the same polarization axis, circularly polarized light having the same polarization axis, or elliptically polarized light having a polarization axis which differs by 90°, the transmittance of the polarizer plate on the emission side is determined by the polarization state, and a display of a gray scale is obtained.
As described, in the LCD of TN liquid crystal, a degree of tilt, from the pretilt angle, of the direction of the major axis of the liquid crystal molecule with respect to the direction of the plane of the substrate is controlled and the slope of the liquid crystal molecule with respect to the viewer when the TN LCD is viewed from the upper right side of the figure significantly differs from the slope of the liquid crystal molecule with respect to the viewer when the TN LCD is viewed from upper left side, as shown in FIG. 1A. Therefore, TN liquid crystal is known to have a large viewing angle dependency and frequent occurrence of coloring and inversion of display. In other words, the TN liquid crystal is known to have a narrow viewing angle which allows view of a normal display.
In order to enlarge the angle of view, Japanese Patent Laid-Open Publication No. Hei 7-311383, for example, proposes dividing the alignment direction of the liquid crystal in one pixel region, that is, formation of an orientation controller in a pixel and division of the direction of the major axis direction of the liquid crystal molecule (liquid crystal director) in a pixel region.
In the VA liquid crystal, on the other hand, as shown in FIG. 1B, the initial alignment is along the direction of normal of the substrate 100, and the difference in the angle of slope of the liquid crystal molecule with respect to the direction of normal is small between a case when the display is viewed from the upper right of the drawing or from the upper left of the drawing. Therefore, compared to the TN liquid crystal, the viewing angle dependency is fundamentally low. In other words, the VA liquid crystal has a characteristic of a wide angle of view. In the VA liquid crystal, however, the direction of the tilt of the liquid crystal molecule from the vertical direction (alignment vector) is not uniquely determined when the voltage is applied, and there is a problem in that a boundary between regions of different alignment directions within one pixel region (disclination line) is not fixed. When the position of the disclination line differs depending on the pixel or changes as time elapses, non-uniformity in display or the like occurs and the display quality is degraded.
In consideration of this problem, references such as Japanese Patent Laid-Open Publication No. Hei 7-311383 disclose provision of the orientation controller in one pixel to fix the disclination line on the orientation controller also in the VA liquid crystal, so that the viewing angle is further enlarged and the display quality is improved.
With the orientation controller as described above, the direction of the initial alignment of the liquid crystal molecule can be controlled so that the occurrence of the disclination line at a random position is prevented and the viewing angle can be enlarged. However, there is a strong demand for further improvement in the display quality and further improvement in responsiveness.